Gemara

28a (Mishnah): If Reuven's jug broke in the Reshus ha'Rabim, and Shimon slipped on the water or was damaged by the shards, Reuven is liable;

R. Yehudah says, if he intended, he is liable. If not, he is exempt.

29a (Abaye): Chachamim (R. Meir) and R. Yehudah argue in two cases: at the time of falling, and afterwards: R. Meir holds that at the time of falling, he is negligent. R. Yehudah says that he is not negligent. ('Intent' is when he broke the flask b'Mezid. Without intent is when he tripped and it broke.)

31a (Mishnah): If two potters were walking one after the other, and one tripped, and the other tripped over him, the first pays the damages.

(R. Yochanan): Our Mishnah is even like Chachamim, who say that one who trips is not negligent, and is exempt. He is liable, for he should have gotten up.

(Rava): Reuven pays the damages to Shimon, both bodily damage and property damage. Shimon pays to Levi only bodily damage, but not property damage.

Question (Beraisa): Every one of them is liable for bodily damage, and exempt for property damage. (This applies even to the first!)

Retraction (Rava): Rather, Reuven pays bodily and property damage. Shimon pays only bodily damage, for a pit is exempt for Kelim.

Question: This is like Shmuel, who says that every obstacle is like a pit. Rav says that an obstacle is like a pit only if the owner was Mafkir it. How can he answer?

Answer: Really, it is like Rava said initially. Rav Ada bar Minyomi taught that the Beraisa exempts everyone for damage to Kelim due to his property.

Rishonim

Rif: R. Yochanan obligates even according to Chachamim, who say that one who trips is not negligent, for here he should have stood. Since he did not, he is not considered Mezid

Rif: Rava taught that Reuven pays to Shimon bodily and property damage. Shimon pays to Levi only bodily damage, but not property damage.

Rosh (ibid.): The Rif explains that the first, who fell by himself, is not so distracted by his fall. He should have stood or warned the other, so he is liable like a man who damages. The second, who tripped on the first, fell harder. He is more distracted by his fall. It suffices to obligate him like a pit for not standing.

Beis Yosef (CM 413 DH v'Rav): The Rosh explains that the Rif obligates Reuven for damage to Shimon's body and property due to Reuven's body. I say that the Rif obligates for damage to Shimon's body due to Reuven's body or property, but totally exempts for damage to Shimon's property. We need not say that 'I did not dig this pit' refers to the Beraisa above; it refers to Rava's law.

Gra (4): It is still difficult, for the first explanation holds that there is payment for damage to Kelim, but the Rambam and Shulchan Aruch exempt!

Beis Yosef (DH u'Mah she'Chosav ha'Rosh): The Rosh said that the Rif explains that the first, who fell by himself, is liable for not standing, like a man who damages. This is wrong. The Rif (14b) asked why one who did not clear away his jug that broke is exempt, but here, he is liable for not standing. He answered that one is Mafkir the shards and water. One is not Mafkir his body, so he is liable, even if he fell b'Ones. This obligates him only like a pit.

Rambam (Hilchos Nizkei Mamon 13:8): If two potters were walking, and Reuven tripped, and Shimon tripped on Reuven, if Reuven could have stood, but did not, he is liable for damage to Shimon. Even though he was Anus at the time he fell, he could have stood. If he could not have stood he is exempt, even though he could have warned Shimon, since he is distracted with himself.

Rambam (9): Reuven pays the damages to Shimon's body. He is exempt for damage to Shimon's Kelim, for every obstacle is a Toldah of a pit, which is exempt for Kelim.

Rambam (10): If Reuven tripped, and Shimon tripped on Reuven, and Levi on Shimon, and each of them could have stood, Reuven pays the damages to Shimon's body, whether he was damaged on Reuven's body or Reuven's load. Shimon pays the damages to Levi's body due to Shimon's body, but not due to Shimon's load. Shimon can say 'I did not dig this pit (my load)', for Reuven made Shimon fall. If they warned each other, all are exempt.

Rosh (ibid.): The primary Perush obligates Reuven for damages to the Shimon's body and Kelim. One who trips is negligent, so he is liable like one who damages. He is liable for damage to Shimon due to his Kelim, like a pit, even if he was Mafkir them, for this resulted from tripping through negligence. Shimon, who tripped on Reuven, is not negligent for falling. Even so, he is liable like one who damaged, for he should have stood or warned Levi. He is exempt for damage due to his shards, for presumably he was Mafkir them. We ask from a Beraisa that exempts everyone for monetary damage, and explain Rava differently, like Chachamim, who hold that one who trips is Ones; he was Mafkir his property after falling through Ones. Reuven is liable for damage due to his body, for he should have stood or warned. Shimon is liable only for damage of his body, which is like a pit, to Levi's body. Shimon, who tripped on Reuven, is more Anus than Reuven. This is a poor answer; they are equally negligent for not standing or warning. We challenge this, for it is unlike Rav. Rav Ada defends the original explanation. The Beraisa exempts for monetary damage, i.e. of everyone. For Reuven, this is only when his Kelim damaged Kelim. He did not say 'Ela', for he did not retract regarding Shmuel, only regarding Rav. In any case, the previous answer was difficult. Since we must explain Rav like Rav Ada, we also explain Shmuel this way. Also, why should we explain Rava's words in two ways?!

Rosh: Rava did not establish the Mishnah like R. Meir because he holds like him. Rather, Rava holds like Shmuel, that every obstacle is a pit, and we cannot establish the Mishnah like this, for it obligates Reuven for damage to Shimon's Kelim. Shimon was partially negligent for tripping over Reuven. We join this to his negligence for not standing or warning, and he is like one who damaged. Chachamim hold that one who trips is Ones, so Shimon is considered a pit. The Mishnah discussed potters and glassmakers, Kelim that break easily, to teach that we discuss Chiyuvim for Kelim. R. Yochanan even Chachamim obligate for not standing. If this is like Rava, he is liable only for damage to others' bodies. However, R. Yochanan holds that the Mishnah mentioned potters to teach that they carried a heavy load and are prone to trip.

Poskim

Shulchan Aruch (CM 413:1): If Reuven tripped and fell, and Shimon tripped on Reuven, if Reuven could have stood, he is liable. If he could not have stood, he is exempt, even though he could have warned Shimon, since he is distracted with himself. He is liable for damage to Shimon's body, but not for damage to Shimon's Kelim. One is exempt for damage of a pit to Kelim. Every obstacle is a Toldah of a pit. Shimon pays the damages to Levi's body, only if he was damaged on Shimon's body, but not if he was damaged on Shimon's load.

Drishah (1 DH Hinei): The Rif brought Rava's teaching to be the Halachah. We conclude like Rava's first Perush. However, the Rosh says that that Rava established the Mishnah like R. Meir, that one who trips is negligent, so he is like one who damages, and his load is like a pit. We rule like Chachamim, that one who trips is Ones, and like Shmuel, that every obstacle is a pit. Therefore, his body is like a pit, so he is liable only for damage to others' bodies. He is exempt for his load, for he was Mafkir it due to Ones. The Rif holds that Chachamim agree that he is negligent for not standing, even though tripping is Ones. Therefore, the Halachah follows Rava's first Perush. This is how the Tur explains the Rif. The Rosh holds that the Rif considers Reuven to be like one who damages, but he is exempt for his load, since one who trips is Ones. His negligence of not standing does not obligate him for his load. Rava obligates property damage to Shimon, i.e. of Shimon's property through Reuven's body.

Shulchan Aruch (2): If Reuven tripped, and Shimon tripped on Reuven, and Levi on Shimon, and each of them could have stood, Reuven pays the damages to Shimon's body, whether he was damaged on Reuven's body or Reuven's load.